1. Field of the Invention
This invention relates to an alternating current generator for a motor vehicle having field cores equipped with claw poles and having permanent magnets disposed between adjacent claw poles.
2. Related Art
A synchronous generator ("alternator") including Lundell-type field cores, i.e., field cores with claw poles, wound with field coils, on a drive shaft is generally used as an alternating current generator for a motor vehicle.
Japanese Patent Laid-Open No. S61-85045 discloses interposing permanent magnets in spaces between neighboring claw poles on field cores with claw poles, and magnetizing these magnets in a direction opposite the magnetic flux leakage between the claw poles in order to reduce the magnetic flux leakage. Such field cores with claw poles are described as magnet type field cores hereinafter.
For an alternating current generator for a motor vehicle, there is a strong demand to maximize the output/weight ratio, i.e., the output/volume ratio, so that there has been a certain optimum style in the Lundell-type field core. The field coil of this type is described below as a non-magnetic field core because it has no permanent magnets disposed between the claw poles.
In general, maximum output I of a generator is determined by the following equation (1). EQU I=K.sub.o -D.sup.2 L-Bg-(ac) (1)
Here, K.sub.o represents a constant depending upon the drive pulley ratio, coil coefficient or armature and other factors, D.sup.2 L represents the energy conversion space volume, which is determined by the outside diameter D of rotor and the thickness or axial length L of the stator core (armature iron core), Bg is the space or air gap magnetic flux density and ac is the ampere conductor applicable to the inside diameter section of the armature, i.e. the ampere turn/.pi.D (A/mm). Bg is proportional to the ratio of effective magnetic flux .PHI.g out of the generated magnetic flux .PHI.s, i.e the magnetic efficiency .eta.. Furthermore, the ampere conductor ac depends mainly on the number of windings of the armature coil, and, as it is understood from the fact that where there are more poles, the same voltage can be obtained with a lesser number of windings. Ampere conductor ac is also proportional to the pole number P. In other words, the output I can be expressed as follows in equation (2): EQU I=K.sub.o '-D.sup.2 L-.eta.-(P/D) (2)
Thus, it is well known that I is proportional to the magnetic efficiency .eta. and (P/D) as well as the physical structure.
Although equation (2) suggests that the output should increase as the pole number P increases as long as the physical structure is the same (D.sup.2 L and D are constant), it is known qualitatively that the optimum pole number P must be determined as a matter of course, as the magnetic efficiency .eta. actually drops. See the characteristic example provided in FIG. 5 illustrating the situation "Without magnet".